1. Field of the Invention
The present invention relates generally to golf balls containing at least one layer made from a rubber composition comprising a nucleating agent. More particularly, the nucleating agent is a benzenetrisamide compound. The composition may be used to form any layer in the golf ball structure such as, for example, a core, intermediate layer, and/or cover. Preferably, the composition is used to form a core having improved resiliency and other physical properties.
2. Brief Review of the Related Art
Nucleating agents are commonly added to crystallizable thermoplastics, such as polyoelfins, to make products used in various industrial applications. The nucleating agent provides nucleating sites for the thermoplastic material to crystallize from the melt. In this manner, the nucleating agents help control the crystallization rate, crystal size, and other crystalline properties of the material. Films, sheets, molded parts, and other products having improved chemical and physical properties may be made from the crystallizable material. For example, nucleating agents may be added to a crystallizable polypropylene resin to make a film. The stiffness, surface hardness, and scratch-resistant properties of the resulting polypropylene films may be increased.
McCullough, Jr, et al., U.S. Pat. No. 5,362,782 discloses polypropylene impact copolymer compositions containing a nucleating agent such as 50 to 2000 ppm sodium benzoate. This increases the stiffness and notched Izod impact strength of the composition making it useful in the production of molded and extruded articles, shaped containers, and films having good clarity.
Chatterjee, U.S. Pat. No. 5,674,630 discloses polypropylene impact copolymer compositions containing a homopolymer phase of predominantly homopolymeric polypropylene, a copolymer phase of copolymerized ethylene and propylene, and a nucleating agent. The compositions have a rubber content (Fc) in the range of 25 to 45% by weight, a crystallization temperature in the range of 122° to 132° C., a melt flow in the range of about 7 to about 60 dg/min, and a ratio of the intrinsic viscosity of the copolymer phase to the intrinsic viscosity of the homopolymer phase of 1.4 to about 1.9. The polypropylene impact copolymer compositions may be used to form cast films.
In recent years, multi-piece solid golf balls have become more popular for several reasons, including, ease of manufacturing, material costs, ball properties, and ball playing performance. Basically, a two-piece solid golf ball includes a solid inner core protected by an outer cover. The inner core is made commonly of a rubber material such as natural and synthetic rubbers, styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene). The outer cover is made commonly of a thermoplastic such as ionomer resins, polyamides or polyesters; and thermoplastic and thermoset polyurethane and polyurea elastomers. Referring to FIG. 1, a golf ball (10) having a conventional two-piece design is shown. The ball (10) includes an inner core (12) and outer cover (14). As new materials and manufacturing processes have become available, three-piece and four-piece solid golf balls have been introduced. Different materials can be used to impart specific properties and features to the ball. For example, FIG. 2 shows a conventional three-piece golf ball (20) having an inner core (22), at least one intermediate layer (24) surrounding the core, and an outer cover (26). The intermediate layer (24) may serve as a water vapor barrier to prevent moisture from penetrating into the underlying core (22). In other instances, a four-piece solid golf ball (30) having an inner core (31) and surrounding core layer (32), as shown in FIG. 3, is made. The ball (30) further includes an intermediate layer (34) and outer cover (36).
The core is the primary source of resiliency for the golf ball and is often referred to as the “engine” of the ball. The resiliency or coefficient of restitution (“COR”) of a golf ball (or golf ball sub-component such as a core) means the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize the COR under these conditions. Balls (or cores) with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance. In general, the COR of the ball will increase as the hardness of the ball is increased. The test methods for measuring the COR are described in further detail below. In some conventional golf balls, materials are used to increase the hardness of the core so that the resiliency of the core is increased, and this, in turn, causes the compression of the core to increase.
In general, compression refers to the deflection that a golf ball (or golf ball component such as a core) undergoes when subjected to a compressive load. Several different methods may be used to measure compression of the golf ball and its components. The compression is commonly measured using Atti or Riehle compression gauges. In the Atti method, as described further below, a piston is used to compress a ball against a spring. The travel distance and load of the piston are fixed and the deflection of the spring is measured. If the core is relatively soft, the spring will deflect only by a minimal amount. On the other hand, if the core is relatively hard, the spring will deflect a significant amount. The test methods for measuring compression are described in further detail below.
Cores having a higher compression are harder and tend to have good durability, toughness, and impact strength as well as resiliency. Players may achieve greater flight distance when using such golf balls, which is particularly desirable when hitting the ball off the tee. However, high compression balls are relatively stiff and this may have a detrimental effect. Players tend to experience a harder “feel” when their club face makes contact with such golf balls. The player senses less control, and the harder ball tends to have low initial spin. The sensation of striking the ball is generally less natural and comfortable with “hard” golf balls versus “soft” golf balls. With a golf ball having a softer feel, the player can place a spin on the ball and better control its flight pattern. The softer golf ball feels more natural. The player senses more control, and the softer ball tends to have high initial spin. This is particularly desirable when making approach shots near a golf hole green. Skilled players can place a back-spin on such balls so that they land precisely on the green.
It would be desirable to develop a core material that provides enhanced resiliency along with a soft feel to the golf ball. The core material should have good durability, toughness, and impact strength. The core material should have high resiliency and COR so that a player can drive the ball long distances. The core material, however, should not be so stiff that playing performance properties such as feel, softness, and spin control are sacrificed. One objective of the present invention is to develop a core material having an optimum combination of hard and soft properties. Improved resiliency, durability, toughness, and impact strength are some desirable hard properties, while a better feel and spin control are some desirable soft properties. The present invention provides a core material and the resulting golf ball having these properties as well as other advantageous features and characteristics.